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ANTENNA Tuner Correct Specs But Misleading

Discussion in 'Ham Radio Discussions' started by KC3EPA, Apr 15, 2018.

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  1. W4NNF

    W4NNF XML Subscriber QRZ Page

    They are not any kind of a bandage. They allow the transmitter to be properly matched to the antenna feedline. They are not designed to do anything about reflected power, which will eventually be radiated--that which is not lost as thermal energy. There are many factors to consider. Not least of which is that a resonant antenna is hardly a panacea and not always a good antenna. ;)
     
    NQ9L likes this.
  2. K1FBI

    K1FBI Premium Subscriber QRZ Page

    Enter some numbers here and see how your theory plays out.
    http://www.qsl.net/co8tw/Coax_Calculator.htm
     
  3. W0VRA

    W0VRA Ham Member QRZ Page

    Thanks!

    @KK5JY gave me some very helpful explanation, and I had misunderstood the subject when I wrote that.
     
    K1FBI likes this.
  4. WA9SVD

    WA9SVD Ham Member QRZ Page

    More correctly, they allow the transmitter to (hopefully) be matched to the tuner. Itr then is the job for the tuner to find a reasonable match to the impedance presented at its output. If that is 1000 feet of RG-58 @29 MHz, the match seen by the tuner may seem very good indeed. IF it is 1' or less, AT the antenna, the match will stil be bad, but hidden by coax loss. ALL a tuner really does is make a transmitter reasonably happy, with a low SWR. It does NOT improve the SWR from tuner >transmission line > antenna.
     
  5. W4NNF

    W4NNF XML Subscriber QRZ Page

    And that is all it is supposed to do. The purpose of a transmatch/antenna tuner to match the transmitter to the antenna system, allowing power to be transferred to the antenna. Not to somehow "fix" SWR. The performance characteristics of the antenna system are a whole other subject. High SWR is most assuredly an indication there will be a poor match between transmitter and antenna, but it doesn't mean the transmatch fixes that or is intended to do so. :)
     
  6. WA9SVD

    WA9SVD Ham Member QRZ Page

    A bit of a fallacy.
    A transmatch is to make the transmitter look "happy." ANY impedance transformation (and thus losses) occur in the transmatch/tuner and the antenna. If there is a huge coax loss (either intrinsic or due to high VSWR) by the time a signal gets to the antenna, it's the tuner/feedline fault, although the transmitter would never know it. A transmatch or tuner does NOT ultimately "match a transmitter to an antenna system." It allows a transmitter to output efficiently, no matter how inefficient or effectively it provides an antenna; a transmatch could easily be close to 100% efficient into a 50 Ohm dummy load, but the transmitter would (or could) easy see a 1.0:1 (or close) SWR, Increase that to a 100 Ohm dummy load, and the transmitter may well see near a 1:1 SWR, but he tuner is not, and loss will ensue.
     
    W2AI likes this.
  7. NQ9L

    NQ9L Ham Member QRZ Page

    I wanted to offer a word of encouragement to anybody using antenna tuners (couplers) in a mobile situation. To me, the meaningful distinction between a tuner and a coupler is where it is located between the transmitter and antenna. As has already been discussed, a tuner matches the complex impedance of the feedline and antenna together. As I understand, the ~30 pF shunt capacitance per foot of coaxial cable makes for high losses in a mismatch. With a coaxial feedline between the tuner and the antenna, I wouldn't want to tune a mismatch much in excess of 4:1. However, if the coupler is located at the feedpoint, then very non-resonant antennas can be used. As has already been mentioned, this is the approach used by many professional users of HF.

    There are some important considerations for this type of antenna system though. Coax is out of the question, so we have to use a single-wire feeder. One must remember that the antenna effectively starts inside the antenna coupler, which means that any length of feeder to the antenna mount is also part of the antenna. This run must be kept as short as possible for obvious reasons, especially if the coupler is inside the vehicle. To help this, an antenna coupler could be installed in a weatherproof enclosure on the outside of the vehicle, just at the antenna base. The open wire feeder should be kept away from grounded metal, especially parallel runs which increase shunt capacitance at the antenna base.

    The other major consideration is that extreme voltages can be present on the output of an antenna coupler: voltages which can arc over mounts designed for resonant antennas. The little nylon washers used by most mounts are not up to the job. Breedlove sells an "HF insulator" for this exact purpose. The last major consideration I'll mention is that the tuner/coupler ground connection must be extremely solid and of low impedance at HF frequencies: for example, my AH-4 has a 4" wide strap of copper flashing from the ground post to the vehicle body. Bonding can help further mitigate ground losses. Again, the coupler is mounted right at the feedpoint.

    So why operate a non-resonant antenna? I hear some hams brag about how they only use resonant antennas or spend time pruning elements to length. Resonant antennas, monoband and multiband have their time and place, but so do non-resonant antennas. The easiest way to avoid problems, after following the above guidelines on coupler location and installation, is to use an unloaded whip. As has been alluded to, antennas with loading coils and antenna couplers are a mixed bag of tricks. Just to give an idea of some of the things that can happen for example: Using a resonant, loaded antenna on a higher band than it is loaded for can cause the loading coil to act as a choke. Using a resonant, loaded antenna on a lower band than it is designed for can have the effect of inducing extreme circulating currents in the loading coil. These effects are largely concerned with the self resonant frequency of the loading coil.

    This being said, there are ways to increase the efficiency of a coupler+whip, which is comparable in performance to a base loaded antenna. The top of the antenna can be loaded: use a cap hat to place most of the capacitance on the end of the antenna and move the current node upwards. A "correctly designed" loading coil can be used in a non-resonant antenna, but this may be beyond the scope of my post (it mostly has to do with withstanding high electromotive stress). These ways of improving current distribution in the antenna can help the radiated signal, but ground losses always dominate the question of efficiency in any mobile setup.

    There are many ways to skin this cat -- this is just an example of one well-explored approach. It has taken me almost all of my several short years as a radio ham to get my mobile HF working well enough for me to consider it "okay". I encourage anybody trying mobile HF to keep at it, experiment, and read. Regarding the comment earlier in this thread about keeping mobile operations to VHF and UHF, I have one word: NO. Judicious compromise is the name of the game.

    Also, I have successfully used one of the MFJ tuners with 6-1600 ohm range (the PCB from the 929) in this type of setup. With robust ground and a 102" whip in the middle of the roof on a ceramic insulator, the 929 loaded 20 and up. I think the MFJ tuners only have around 24 uH of inductance, which is usually not enough to load up a short whip on the low bands.

    Just my 2.5 cents,
    Happy mobiling.
     
    Last edited: Apr 16, 2018
    KB4QAA likes this.
  8. W4NNF

    W4NNF XML Subscriber QRZ Page

    "Happy" has nothing to do with it. My transmitter can no more be happy than my toaster can be. :cool:

    A fallacy? Not at all. Actually, I believe we are (mostly) in agreement. The purpose of a transmatch is to present an impedance the transmitter can work with without destroying its finals, folding back power, or transferring an ineffective amount of power to the antenna system. What happens after the transmitter depends on the condition of the antenna system and to some extent the design of the tuner. As I said, earlier, I can load up a lawn chair with a good enough tuner, but that doesn't mean it will be effective. On the other hand, I can also load up a non-resonant antenna--which can be quite effective indeed.

    HOWEVER, the idea that a transmatch is some kind of a Band-Aid "fooling" the transmitter or operator or both is silly. ;)
     
    W2VW and NQ9L like this.
  9. VE3DDL

    VE3DDL Ham Member QRZ Page

    if its on the trunk lid run ground strap too back seat frame or good ground most trunks are on plastic bushing .or the108 whip same thing need good ground to body of car like grounding straps used by bell and hydro
     
  10. K0RGR

    K0RGR XML Subscriber QRZ Page

    This is much like people trying to 'stretch' a CB antenna to work on 20 meters - it can be done, but efficiency suffers.

    The 40 meter Hamstick does not suck, at least not as much as the 75 meter one does. However, it's not the most efficient antenna. But that won't stop you from making contacts with it. I used Hamsticks for many years with decent results. I've also used Hustlers with great success. But, once I tried a large-coil screwdriver, I knew the Hustlers and Hamsticks were second best.

    I've used an autotuner to stretch a Hustler or Hamstick from the phone band down to the CW band, and it worked, but again, not as well as adding an alligator clip to the antenna to make it longer.
     

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